Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones and security cameras, as well as medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular complementary metal-oxide-semiconductor (“CMOS”) image sensors (“CIS”), has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors. Miniaturization also contributes to cost reduction of image sensors.
One field of application in which size and image quality is particularly important is medical applications (e.g., endoscopes). For medical applications the image sensor chip must typically be small while providing a high quality image. In order to achieve these characteristics, for a given chip size the photosensitive apertures should be as large as possible while peripheral circuitry should be as limited as possible.
The pixel (picture element) fill factor denotes that fraction of the area of a pixel that is sensitive to light. Pixel pitch is the physical distance between the pixels in an imaging device. Pixel fill factor has become smaller as pixel pitch has been reduced because the active circuit elements and metal interconnects consume increasing amounts of area in each pixel. One way to address the loss of fill factor is to use a microscale lens (microlens) directly above each pixel to focus the light directly towards the photosensitive portion of the area within the pixel. Another way to address the loss of fill factor is to use backside-illuminated (“BSI”) image sensors which place the active pixel circuit elements and metal interconnects on a frontside of an image sensor die, and a photosensitive element within the substrate facing a backside of an image sensor die. For BSI image sensors, the majority of photon absorption occurs near the backside silicon surface. However, a solution that provides larger individual pixel area on the same silicon area would improve BSI image sensors as well as frontside illuminated image sensors.